1. Field of the Invention
The present invention relates generally to a lighting device such as a headlight, an auxiliary headlight or the like mounted on a vehicle. More particularly, the present invention relates to improvement of a reflector for the lighting device of the foregoing type.
2. Background Art
To facilitate understanding of the present invention, a typical conventional reflector for a lighting device of the foregoing type will be described below with reference to FIG. 3 to FIG. 5.
FIG. 3 shows by way of perspective view the structure of a reflector 91 for a lighting device 90 mounted on a vehicle (not shown). The reflector 91 is molded of a synthetic resin and includes a reflective surface 91a having a predetermined configuration composed of, e.g., a revolving parabolic plane, and a light source 92 such as a halogen lamp or the like is disposed at the position substantially coincident with a focus of the reflective surface 91a. In addition, a lens 93 is arranged ahead of the reflector 91 so that the lighting device 90 exhibits desired light distribution properties. For the purpose of adequately coordinating the configuration of the reflector 91 with the design of a vehicle, upper and lower parts of the reflector 91 are cut out to form an upper flat surface 91b and a lower flat surface 91c both of which extends in parallel with an optical axis Z of the lighting device 90.
With respect to the lighting device 90 constructed in the above-described manner, it has been found that the light distribution properties immediately after it is turned on vary from the light distribution properties when it is continuously turned on for a predetermined period of time, e.g., ten minutes. This variation of the light distribution properties of the lighting device is exemplified with reference to the light distribution properties employable in case that a light beam is irradiated toward a vehicle running in the opposite direction as illustrated in FIG. 4 and FIG. 5. Specifically, FIG. 4 shows light distribution properties H0 of the lighting device 90 immediately after the light source 92 is turned on, and FIG. 5 shows light distribution properties H1 when the light source 92 is continuously turned on for ten or more minutes. In contrast with the light distribution properties H0, the light beam distribution properties H1 has a separate part H1a which represents that a part of the reflected light beam is slantwise upward irradiated by angle of about 0.6 degree relative to a horizontal plane H.
The appearance of the separate part H1a in that way leads to a problem that a dazzling light beam is generated in the practical use wherein the light source 92 is normally continuously turned on for a certain time. In addition, there arises another problem that visual recognizability of the lighting device 90 is degraded because of undesirable deformation of the contour representing the light distribution properties. To prevent the dazzling light beam from being irradiated from the lighting device 90, it is necessary that when the lighting device 90 is mounted on a vehicle, a controlling unit (not shown) for the lighting device 90 is preliminarily adjusted such that the optical axis of the lighting device 90 is automatically corrected after the light source 92 is continuously turned on for a certain time, e.g., 10 or more minutes, resulting in an idling time being unavoidably required.